The present invention relates generally to an ion-sensitive structure and to a method for producing the same. The ion-sensitive structures can be used, for example, in ion-sensitive field effect transistors (ISFETs), ion-sensitive, capacitively readable EIS sensors or LAPS sensors. In particular, the present invention relates to a chemically stable multilayered structure having doped zones.
In microsystem technology of semiconductor technology, chemically stable surfaces are necessitated as chemical protection for sensor or actuator surfaces for sensors and actuators in the different fields of application, for example environmental technology, process monitoring, food technology as well as biochemistry or medical technology. When the advantages of a chemical protective layer can also be used as sensors for chemicals, this is a particular advantage for sensor technology, for a low long-term drift of the sensor in connection with a reasonable price. In particular, the above stated electrolyte-insulator-semiconductor structures can be stated as fields of applications, which can also be configured specifically as ion-sensitive field effect transistors or ion-sensitive sensors with EIS or light-addressable ion-sensitive sensors with EIS (LAPS).
Silicon is mostly used as a semiconductor of the EIS structure. All other semiconductor/insulator combinations are mostly electrically not as stable or the necessitated electrical long term stability and accuracy can only be obtained with increased effort. Further, in the field of pH measuring arrangement, glass electrodes are known. However, the disadvantages of glass electrodes with respect to semiconductor sensors are sufficiently known, as well as how the EIS structure is used for analysis. Meanwhile, semiconductor solid body electrodes are used for PH measurement in industrial measurement technology, not only for reasons of risk of glass breakage.
The EIS sensor without FET structure has the advantage that no topology edges representing a risk for chemically aggressive media interfere with the surfaces as it is still the case in the ion-sensitive field effect transistor. However, such a structure can only be read out via capacitance measurement necessitating a measurement cell with a Faraday cage. Due to recurring topology edges, a closed chemically stabile protective layer is even more important.
For producing hydrogen ion-sensitive layers, different materials have been described, such as Ta2O5 [2,3], AL2O3 [4], TiO2 [5], HfO2 [8] and simple metal nitrides [10] or double metal oxide mixtures, such as TaAlO and ZrAlO [6] or combinations of two different amorphous metal oxide layers [11] and diamond like carbon (DLC) [9] that are relatively stable, apart from few exceptions [7], in a thermo dynamical manner in aqueous alkaline and oxidative environments. In particular, the introduction of metal oxides obtained significant improvements of pH sensor characteristics as well as long-term stability and drift of these sensors, in particular with respect to Si3N4-ISFETs.
Conventionally, metal oxides have been deposited and only tempered such that their amorphous structure is maintained. Thus, the same are chemically less stable and more light-sensitive than their crystalline counterparts in bulk chemistry and are subject to planar etching at increased temperature in corrosive media.
Crystalline simple metal oxide layers are chemically more stable than amorphous layers, since crystallites have a high density comparable to the bulk material. The weaknesses of these layers are the grain boundaries of the crystallites, in particular the vertical ones allowing pore etching and consequences of sub-etching at increased temperature in aggressive media. Thus, stable operation of sputtered and crystallized Ta2O5-ISFETs can only be ensured up to a pH value of approximately 12 and a temperature up to 75° C.
In multilayered polycrystalline metal oxides of one substance type, the vertical grain boundaries are only insufficiently interrupted since the crystals of the first layer act as growth seeds for the second layer.
Multilayered amorphous metal oxides of different substances are chemically more stable than simple ones, but the problems accompanying amorphous structures could not be satisfactorily solved.
The crystallizations of these multilayers of different metal oxide layers are, as can be expected, again chemically more stable, but the interfaces between the substance types result in typical interface problems. These are poor reproducibility of the flat band voltage and so-called charge trapping.
The deposition of metals with subsequent thermal oxidation [12] results, for kinetical reasons, in poor quality of the semiconductor/insulator combination, such as Si/SiO2, since the metal has a reducing effect on an applied insulator (e.g., SiO2) and thereby partly removes the oxygen from the insulator layer. This results in fixed charges in the gate insulator causing operating point shifts of the sensor and drifts by trapping processes. The trapping processes result in leakage currents which affect the chemical stability of the whole insulator stack.
Pure metal oxides have the features characteristic for its substance which can significantly differ from other metal oxides, such as HfO2. Currently, HfO2 is more frequently used due to its excellent chemical and physical characteristics as crystalline bulk material, both in sensor technology as well as in highly integrated microprocessor technology. Such features are the extremely high electric isolation values as well as the extremely high chemical resistance in aggressive media up to the commonly highest applied temperature. However, as a layer, it has a distinctive structure and mainly vertical grain boundaries [14].
The same responds to the temperings necessitated for stabilization and compression with extensive anisotropic thermal expansions which can lead to cracks in the layer at topology edges. Pure metal oxides mostly maintain their features characteristic for their substances even when stacked with others. Packing a thick HfO2 layer between a base and a cover layer of a previously and subsequently deposited different metal oxide, such as Ta2O5, does not change the essential behavior of HfO2. Even when HfO2 is deposited in one or several thin amorphous layers and tempered with Ta2O5, the charges at the interfaces and in the HfO2 layer itself are only difficult to control. Consequences are sensor drifts and light sensitivities that are not so small as they can be adjusted by Ta2O5 in a reproducible manner although the chemical stabilities are much greater [16].
Metal oxynitrides and also double metal oxynitride mixtures such as compounds such as, for example, HfxTayOaNb [15] induce interfaces between the oxynitrid and the SiO2 that are electrically hard to control.